Fluid-translating apparatus

ABSTRACT

A constant pressure variable displacement axial piston pump employing two stroking cams and two banks of opposed pistons mounted in circular array in rotary cylinder barrels positioned on opposite sides of a flat port plate. The stroking cams and piston slippers have arcuate concave cam and convex bearing surfaces, respectively, formed about an axis which intersects and is perpendicular to the axis of rotation of the cylinder barrels. With this construction, each piston will be forced through two strokes per revolution of the cylinder barrels, all radial forces will be in inherent balance, and there will be an area contact between the piston slippers and the stroking cams at all times.

United States Patent Inventor Richard Arthur Wittren Cedar Falls, Iowa AppL No. 767,318

Filed Oct. 14, 1968 Patented Aug. 3,1971

Assignee Deere & Company Moline, 111.

FLUED-TRANSLATING APPARATUS References Cited UNITED STATES PATENTS Primary Examiner-William L. Freeh Altorneysl'l. Vincent Harsha, Raymond L. Hollister,

William A Murray and John M. Nolan ABSTRACT: A constant pressure variable displacement axial piston pump employing two stroking cams and two banks of opposed pistons mounted in circular array in rotary cylinder barrels positioned on opposite sides of a flat port plate. The stroking cams and piston slippers have arcuate concave cam and convex bearing surfaces, respectively, formed about an axis which intersects and is perpendicular to the axis of rotation of the cylinder barrels. With this construction, each piston will be forced through two strokes per revolution of the Re.23,993 5/1955 Henrichsen 103/162 cylinder barrels, all radial forces will be in inherent balance, 1,389,873 9/ 1921 Huh 103/43 and there will be an area contact between the piston slippers 2,453,128 1 H1948 Hautzenroeder 1031162 and the stroking m r times- E\\ O 20 z 2 3 I 8 a no a 26 a1,

1 7 61 5 53 an 5961 11 x 29 a a5 93 55 E l 50 56 94 93 PATENIED AUG 3 I9?! SHEET 1 OF 5 INVENI'OR. R A WITTREN ATTORNEY SHEET 3 OF 5 PATENTEU Am; 3 I971 I N VEN '1 0R. R. A. WITTREN FIG. l2

ATTORNEY PATENTEUAUI; 3|97| 3,596,568

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INVENTOR.

R. A. WITTREN ATTORNEY FLUID-TRANSLATING APPARATUS BACKGROUND OF THE INVENTION have their projecting ends in driving engagement with a cam.

member. Machines of this type, when provided with suitable Yet anotherobject of the present invention is to provide a high-pressure and high-output variable displacement pump which operates at a constant pressure, in which there is a complete balance of all axial and radial force components, in which there is minimum of piston overhang and piston side thrust at the fully retracted position of the pistons, which will operate with a shorter stroke for a given displacement, and in which the'pistons are driven through two strokes for every revolution of the pump while retaining an area contact between the cam and the piston slippers.

Still another object of the present invention is to provide an axial piston pump or motor in which all the radial force comvalving, are well suited for useas hydraulic pumps and motors, I

the machine operating as a pump when relative rotation between the cylinder barrel and cam member is imparted by an external source and as a motor when fluid pressure is applied to the pistons. Therefore, in its most specific form, the present invention relates to fluid-translating apparatus which may operate as a pump or a motor.

In all of the commercially successful axial piston-type pumps and motors, the cam or swash plates have had a substantially planar surface on which thepiston or piston slippers would ride. The planar surface would permit contact between the cam or swash plate and the slippers over a relatively large area so that when the pump or motor was operated under conditions of high pressure, the force between the pistons and the cam plate was distributed over a large area. However, the use of a cam plate having a substantially planar surface had its limitations. For example, the displacement of the pump or motor was limited due to the fact that the piston could only be 'driven through a single stroke per revolution, the side thrust on the pistons limited the permissible length of a piston stroke, and unbalanced radial forces created large bearing requirements.

Attempts have been made to avoid the limitations of cam plates having substantially planar surfaces by providing pumps and motors with sinuous cams. The use of sinuous cams provided for a larger displacement per revolution and also balanced the radial forces, but the stroke length of the pistons was still limited due to the side thrust exerted on the pistons when they were in a partially retracted condition. Also, and most importantly, the sinuous cams did not allow for an area contact between the cam and piston slippers but, due to the fact that the slippers had to follow the serpentine path of the sinuous cam, the contact was limited to a line or point. With a line or point contact, the pump or motor could not operate under conditions of high pressure without damage to the cam or pistons. Examples of pumps having sinuous cams are illustrated in U. S. Pat. No. 2,439,668 issued to A. A. Mercer on Apr. 13,1948, No. 2,965,304 issued to W. S. Fallon on Dec. 23,1958 and No. 3,079,869 issued to H. M. Purcell on March 5,1963.

SUMMARY OF THE INVENTION The primary object of the present invention is to provide an axial piston pump or motor in which each piston has two strokes for each full revolution of the pump or motor and in which there is an area contact between the piston slippers and the cam through the entire revolution. This object is accomplished by providing a cam having a surface formed as a portion of a cylinder, that is, an arcuate surface which is formed about an axis which intersects and is perpendicular to the axis about which the cylinders are positioned.

Another object of the present invention is to provide an axial piston pump or motor which inherently provides minimum piston side thrust at maximum protrusion of the pistons from the cylinder block while maximum side thrust occurs when the pistons are in their innermost position, thus allowing longer piston strokes and higher displacement. This object is obtained by providing a cam having a concave arcuate cam surface formed about an axis which intersects and is perpendicular to the axis about which the cylinders are positioned.

ponents arebalanced so that the cylinder barrels can be loosely mounted on the drive shaft and will be self-aligning.

The above objects and additional objects and advantages will. become apparent along with the details of construction of a preferred embodiment of the invention from a reading of the following detailed description when taken in conjunction with the accompanying drawings.

In the following description, the device shown in the drawings will be described as a pump, but those skilled in the art will recognize that the details of construction are equally applicable to a motor.

BRIEF DESCRIPTION OF THE DRAWINGS 2-: ofFIG. 1;

FIG. 3 is a top plan view of the slipper retainers or return cams used in the pump illustrated in FIG. 1;

FIG. 4 is a sectional view taken substantially along the lines 4-4 of FIG. 3;

FIG. 5 is an end elevational view of one of the cylinder barrels taken in the direction of the arrows 5-5 of FIG. 1;

FIG. 6 is an end elevational view of one of the cylinder barrel driving flanges taken in the direction of the arrows 6-6 of FIG. 1;

FIG. 7 is an elevational view of the left end of the pump illustrated in FIG. I with the end cover removed and the cams rotated through 22 k";

FIG. 8 is an elevational view of the right end of the pump illustrated in FIG. 1 with the end cover removed and the cams rotated through 22 15;

FIG. 9 is a plan view of the central section of the pump housing;

FIG. 10 is an elevational view of a portion of the central section of the pump housing, taken substantially along the lines 10-10 of FIG. 9;

FIG. 11 is a sectional view taken substantially along the lines 11-11 of FIG. 2; and

FIG. 12 is a top plan view of a portion of a pump stroke control mechanism.

DESCRIPTION OF THE PREFERRED EMBODIMENT The pump illustrated in the drawings has a housing consisting of three sections to facilitate assembly of the pumping mechanism within the housing and includes a main central section 20 and left and right end covers 21 and 22, respectively. The end covers 21 and 22 make a metal-to-metal contact with the central housing section 20 to insure proper location of the end covers with respect to the central section of the housing, and are held in position by a plurality of undisclosed bolts. Suitable packings 23 seal the joints between the end covers and central section of the housing. The central section of the housing is provided with openings 24 which connect with fluid supply lines through standard fittings 25.'The central section of the housing is also provided with an opening 26 which connects with a high-pressure exhaust line through a standard fitting 27. Diametrically opposed from the opening 26, the central housing section is provided with an additional opening which threadedly receives a stroke control valve 28 which will be more fully described hereinafter.

A drive shaft 29 is joumaled in the housing covers 21 and 22 by antifriction bearings 30 and 31. Sinceall the radial and axial force components in the pump are inherently balanced, the bearings 30 and 31 need only be of nominal load rating for accurate radial and axial location of the pumping elements and to improve shaft seal performance. The bearings 30 are roller bearings and carry radial loads which may be caused by the drive means coupled to the projecting end of the shaft. Seals 32 and 33 prevent any leakage of fluid from the housing between the shaft 29 and the end cover 21. The bearings 31 are ball bearings to provide both radial and axial location of the shaft 29 within the housing. The drive shaft 29 is held in proper position with respect to the bearing 31 by nuts 34 andv 35 which are threaded onto the end of the shaft 29 and engage the respective sides of the inner race of the bearing 31, and the bearing 31 is held in proper position against an abutment 36 on the end cover 22 by a cap 37 which is threaded into the end cover 22 and engages one side of the outer race of the bearing 31 and maintains the opposite side of the race in engagement with the abutment 36. A seal 38 prevents any leakage of fluid from the housing between the cover 22 and the cap 37.

A valve plate 39 is mounted in the central part of the chamber formed by the pump housing and encircles the drive shaft 29. The valve plate 39 is mounted for a limited degree of self-alignment by opposed junction blocks 40 and 41. Each of the junction blocks 40 and 41 has a tapered end portion 42 with seats in a tapered recess 43 provided in the perimeter of the valve plate 39. The junction block 40 is held in position by an end portion 44 which extends into the opening 26 provided in the central section of the housing and the junction block 41 is held in position by a portion of the heretofore mentioned stroke control valve 28 which extends into the junction block 41. The tapered joints 42--43 between valve plate 39 and the junction blocks 40 and 41 are constructed to be slightly loose so that limited axial sliding movement can take place between valve plate 39 and junction blocks 40 and 41 and also so that the valve plate 39 is free to rotate slightly about any axis perpendicular to the shaft 29. The limited sliding and rotary movement of the valve plate 39 allows it to move into proper alignment when it is engaged by the cylinder barrels to be described hereinafter. Seals 43a prevent leakage of fluid between the joints 42-43 while allowing the slight movement of the valve plate.

The junction blocks 40 and 41 are hollow and are placed in fluid communication with pressure exhaust ports 45 and 46, respectively, provided in the valve plate 39 by radial bores 47 and 48, respectively. The junction blocks 40 and 41 are also placed in fluid communication with each other by fluid conduits or tubes 49 and 50. Each of the tubes 49 and 50 extends into suitable openings 51 and 52 provided in the junction blocks 40 and 41, respectively, and are secured therein in any suitable manner such as brazing. It can thus be seen that any fluid pressure within the pressure exhaust port 45 is free to flow directly to the high-pressure fluid lines through the opening 26 in the central section 20 of the pump housing and fluid within the pressure exhaust port 46 will act against the end of the stroke control valve 28 and will also flow to the high-pressure line by way of the tubes 49 and 50 and the opening 26.

Rotary cylinder barrels 53 and 54 engage the opposite faces of the valve plate 39 and are drivingly connected to the drive shaft 29 by cylinder barrel drive flanges 55 and 56. The drive flanges 55 and 56 closely fit and are keyed as at 57 to the drive shaft 29. A driving connection is established between the cylinder barrels and the drive flanges by lugs 58 on the cylinder barrels 53 and 54 which project into suitable recesses 59 provided in the outer edge of the driving flanges 55 and 56. In order to hold the cylinder barrels in sealing contact with the valve plate and to hold the valve plate in proper alignment, the drive flanges 55 and 56 are provided with annular pressure chambers 60 which are exposed to the pump discharge pressure. Fluid pressure within the pressure chambers 60 acts against the ends of the cylinder barrels 53 and 54 to urge the cylinder barrels against the faces of the valve plate. Fluid pressure is routed to the annular pressure chambers 60 from the pressure exhaust ports 45 and 46 by way of a radial passage 61 extending through the valve plate 39, an annular clearance provided between valve plate a 39 and a sleeve 62 which loosely encircles the drive shaft and extends between the drive flanges 53 and 54, and an annular clearance between the cylinder barrels and drive flanges. Leakage of fluid between the cylinder barrels and drive flanges from the pressure chambers 60 is prevented by suitable seals 63, and seals 64 prevent leakage of fluid between the sleeve 62 and drive flanges 53 and 54. To insure that the cylinder barrels are held against the valve plate 39 before pressure is developed, small compression springs 580 are installed in blind bores 58b in each of the cylinder drive lugs 58 and bear against the bottom surface of the slots 59 in the drive flanges 55 and 56.

Each of the cylinder barrels contains a plurality of inclined cylinders or piston bores 65 which converge inwardly toward the valve plate and are circumferentially spaced about the axis of rotation of the cylinder barrels. A plurality of pistons 66 are reciprocally mounted within the piston bores 65 and have an end projecting from the end of the bores which is remote from the valve plate 39. Each of the drive flanges 55 and 56 is provided with a plurality of clearance holes 67 which correspond in number and location to the piston bores 65, and through which the projecting ends of the pistons 55 can work. Each of the pistons 66 is hollow to reduce inertia forces and also to facilitate assembly of the piston slippers described below.

Looking again at the valve plate 39, low-pressure supply ports 68 and 69 extend through the valve plate at positions spaced from the pressure exhaust ports 45 and 46, and fluid entering the pump housing by way of the openings 24 is free to flow to the supply ports 68 and 69 by way of milled slots 70 and 71. The valve plate 39 is also provided with a plurality'of through bores 72 which are positioned between each pair of adjacent supply and exhaust ports and which provide for the passage of fluid between opposed cylinders whenever the pump is not in its full stroke condition as will be more fully explained hereinafter. Check valves 73 and 74 are provided in fluid passageways between the bores 72 and the pressure exhaust ports 45 and 46. These valves permit fluid pressure within the cylinders 65 to be exhausted to the pressure exhaust ports 45 and 46 before the cylinders overlie the pressure exhaust ports and hence avoid valve timing problems usually associated with port-type pumps which operate under variable conditions such as speed, pressure, oil bulk modulus and temperature.

A piston slipper 75 is universally mounted on the projecting end of each piston 66 by a pin 76. Each of the slippers 75 is provided with a recess which receives the rounded end of the associated piston and each of the pins 76 has a rounded head which is held within a cam surface 77 on thrust cam members 78 and 79 by inner and outer slipper retainers or return earns 80 and 81 which engage the slippers on the face thereof which is opposite from the face engaging the cam surface. The cam surfaces 77 on the thrust cam members 78 and 79 are formed as a section of a concave cylindrical surface. Specifically, each of the cam surfaces 77 is a concave arcuate surface formed about an axis which intersects and is perpendicular to the axis of rotation of the drive shaft 29. The cam surface 77 could be convex, but would have the disadvantages of requiring larger cylinder barrels and maximizing piston side thrust. The bearing surface 82 of each piston slipper is formed complementary to the cam surface 77, and specifically is a convex arcuate surface which is formed about the same axis about which the associated cam surface 77 is formed. The opposite side of each of the slippers 75 also presents an arcuate surface 83 which engages with arcuate surfaces 84 and 85 on the inner and outer return cams 80 and 81 respectively. The bearing surface 83 is similar to the cam surface 77 in that it is concave and is formed about the same axis as the cam surface 77, but has a radius of curvature less than the radius of curvature of th cam surface 77 by an amount equal to the thickness of the slipper flange. The cam surfaces 84 and 85 are similar to the bearing surface 82 in that they are convex and are formed about the same axis as the bearing surface 82, but have a radius of curvature less than the radius of curvature of the bearing surface 82 by an amount equal to the thickness of the slipper flange. Each of the outer return cams is held in position by two pins 86, the head ends 87 of which engage spaced ears 88 on the outer return earns 81. The pins 86 extend through the thrust cam members '78 and 79 and are retained by snap rings 89. Each of the inner return earns 80 is held in position by a sleeve 90. Each return cam 80 abuts against a shoulder on the sleeve 90 and is held in nonrotatable position on the sleeve 90 in any suitable manner such as a key and slot. Each sleeve 90 also carries the associated thrust cam member which is held in.

nonrotatable position thereon in any suitable manner'such as a key and slot and which is held in axial position thereon by a snap ring 91.

The operation of the pump thus far described is as follows: When the shaft 29 is driven by a' prime mover (undisclosed), rotary motion will be transmitted to the cylinder barrels 53 and 54 by way of the drive flanges 55 and 56. As the cylinder barrels 53 and 54 are rotated, the piston slippers 75 are forced to move about the cam surfaces 77, 84 and 85 which force the pistons 66 through reciprocal strokes. As the cylinders 65 move over the supply ports 68 and 69, the pistons 66 will be drawn outwardly by the return cams 80 and 81 and the resultant vacuum will draw fluid from the supply ports 68 and 69 into the cylinders 65. As the cylinders 65 move away from the supply ports 68 and 69, the thrust cam members 78 and 79 will force the pistons 66 inwardly to force the fluid out of the cylinders 65 and into the pressure exhaust ports 45 and 46. During the inward movement of the pistons 66,' the fluid will first be forced through the bores 72 and check valves 73 and 74 so that as the cylinders 65 move over the pressure exhaust ports 45 and 46, the fluid pressure within the cylinders and the exhaust ports will be substantially equal. The fluid pressure forced into the exhaust ports 45 and 46 will flow through the opening 26 in the central section 20 of the housing to the fluid circuit supplied by the pump.

Due to the fact that the thrust cam members 78 and 79 have cylindrical cam surfaces 77 each piston will be forced through two reciprocal strokes for each revolution of the drive shaft 29 and the pump will therefore have a capacity substantially twice that of a planar cam surface pump of substantially equal stroke. Also, due to the fact that the pistons are driven through two strokes at diametrically opposed positions per revolution, all radial forces acting on the thrust cam members 78 and 79 and the cylinder barrels 53 and 54 will be inherently balanced. Furthermore, since there is a cylinder barrel on each side of the valve plate 39, all of the axial forces acting on the valve plate 39 will be in balance.

Provision is also made for varying the displacement of the pump, and this is accomplished by mounting the thrust cam members 78 and 79 in a manner to permit limited oscillatory rotation (45 maximum rotation per cam). To this end, the sleeves 90 are rotatable about their supporting structure on the covers 21 and 22. By rotating the cam members 78 and 79 through 45 and in opposite directions, the movement of each piston in each opposed set will be opposite to the movement of the other piston in the set. For example, as one piston on one side of the valve plate 39 is driven inwardly by a cam member, the corresponding piston on the other side of the valve plate 39 will be withdrawn by the return cams so that the fluid will be forced from one cylinder 65 on one side of the'valve plate 39 to the corresponding cylinder 65 on the opposite side of the valve plate 39. The bores 72 in the valve plate 39 allow for the passage of fluid through the valve plate 39 during this period when the pump is in the standby condition. If the thrust cam members 78 and 79 are rotated through less than 45, the output of the the pump will merely be reduced.

To insure that the thrust cam members 78 and 79 are rotated in unison and in opposite directions, the thrust cam members 78 and 79 are linked together by levers 91. The levers 91 are journaled at their center on stub shafts 92 mounted in the central section 20 of the pump housing. Each end of both. levers 91 is pivotally secured by a pin 93 within a cylindrical shoe 94 which is slidably received within bores 95 provided in the thrust cam members 78 and 79. To reduce frictional forces tending to oppose rotation of the cam members 78 and 79, thrust bearings 96 are provided between the covers 21 and 22 and the thrust cam members 78 and 79. The bearings 96 transmit the piston force reaction from the thrust cam members 78 and 79 to the piston covers 21 and 22.

In order to maintain a substantially constant discharge pressure, the oscillatory rotation of the thrust cam members 78- and 79 is controlled automatically in response to variations in the discharge pressure. This is accomplished by hydraulic actuators indicated generally at 97 which are under the control of the heretofore mentioned stroke control valve 28. There are two hydraulic actuators 97 positioned 180 apart, and

, each includes an arcuate-shaped cylinder 98 which is mounted on the central section 20 of the pump housing by pins 99 and by fluid line fittings .100. Arcuate piston rods 101 are reciprocally mounted within the cylinders 98 and have their projecting ends connected to the thrust cam member 78 by pins 102. It can thus be seen that if fluid pressure is supplied to the cylinders 98 through the fittings 100, the rods 101 will be forced out of the cylinders 98 and will cause the thrust cam member 78 tobe rotated in a counterclockwise direction as viewed in FIG. 7. Since the thrust cam member 79 is linked to the thrust ca'm member 78 by the pivoted lever 91, the thrust cam member 79 will also be rotated. The degree of rotation of the thrust cam members 78 and 79 is limitedby stops 103 and 104 provided on the central section 20 of the pump housing. The stops 103 and 104 are positioned 180 apart. When the thrust cam member 78 has been rotated through 45, the planar faces 105 on the periphery of the thrust cam member 78 will abut against the planar faces 106 on the stop members 103 and 104. When the thrust cam members 78 and 79 are positioned for full discharge of the pump as illustrated in FIG. 1, the faces 107 on the thrust cam member 78 will engage the ends 108 of the cylinders 98. It can thus be seen that rotation of the thrust cam member 78 is limited to oscillatory rotation between the stops 103 and 104 and cylinders 98.

When fluid pressure is exhausted from the cylinders 98, the cams are rotated in the opposite direction by springs 109 which act between the thrust cam member 79 and spring return stops 110 which are an integral part of the central section 20 of the pump housing-The springs 109 are mounted on spring guides 111 which extend around the outer periphery of the thrust cam member 79.

As previously mentioned, fluid pressure within cylinders 98 is controlled by the stroke control valve 28. The valve 28 includes a valve housing 112 which is threaded into the central section 20 of the pump housing and has an end 113 of reduced size which projects into junction block 41. A lock nut 114 holds the valve housing 112 in position within the pump housing. A sleeve 115 is mounted in the end 113 of the valve housing and reciprocally carries a valve member 116 in a bore extending axially therethrough. The two end portions of the sleeve 115 closely fit the end 113 of the valve housing. Immediately adjacent the end portions, diametrically opposed areas of the sleeve 115 have been milled inwardly as at 117 to provide inner and outer fltiid passage slots which intersect the bore through the sleeve 115-as best illustrated in FIG. 11. The central part of the sleeve 115 has been milled inwardly as at 118 to provide a fluid passage between the end portions of the sleeve 115. The end 113 of the valve housing is also provided withan annular groove 119 around its outer periphery and fluid communication is established between the groove 119 and the interior of t'heend portion 113 of the valve housing by radially extending passages 120. The groove 119 is enclosed by a ring 121 which isprovided with two radial passages 122 which are in communication with the groove 119. Suitable fluid lines (not disclosed) interconnect the passages 122 with the fluid line fittings 100 for the cylinders 98. The end 123 of the valve member 116 is exposed to the fluid pressure within the junction block 41 while the other end of the valve member 116 is of reduced diameter and extends beyond the end of the sleeve 115.

The valve member 116 is normally urged to the left as viewed in FIG. 2 by a spring 124 which acts between a guide member 125 and an end cap 126 for the valve housing 112. The end cap 126 is threaded into the housing 112 and locked in position by a nut 127. The guide member 125 is loosely mounted within the housing 1 l2 and is radially located by the spherical end portion of valve member 116 projecting into a spherical recess provided in one side of guide member 125. Movement of the guide member 125 is limited by a snap ring 128 mounted within the interior wall of the housing 112. The valve housing 112 is also provided with passages 129 which communicate the interior of the housing 112 with the interior of the pump housing for a reason which will be explained in the description of operation.

The operation of the above-described control valve 28 is as follows: When the discharge pressure of the pump exceeds a value detennined by the setting of the control valve 28, the fluid pressure within junction block 41 will act against the end 123 of the valve member 116 and force the valve member 116 to the right, as viewed in FIG. 2, against the biasing force of the spring 124. With the valve member 116 shifted to the right, the fluid pressure within junction block 41 will flow into the bore through the sleeve 115, through the inner slot provided by the milledarea 117 in the sleeve 115, through the passage provided by the milled area 118 on the sleeve 115, through the radial passages 120, into the annular groove 119, through the radial passages 122 in the ring 121, through the undisclosed fluid lines, through the fluid line fittings 100 and into the cylinders 98. Fluid pressure within the cylinders 98 will force the rods 101 outwardly to rotate the cam members 78 and 79. As previously described, the rotation of the cam members will cause a decrease in volume output of the pump. The volume output of the pump will be decreased a sufficient amount so that the output of the pump will not exceed the system demands at rated pressure.

If the discharge pressure of the pump falls below the value determined by the setting of the control valve 28, the spring 124 will force the valve member 116 to the left as viewed in FIG. 1, and the fluid pressure withincylinders 98 will be exhausted to the interior of the pump housing by way of the outer slot provided by the milled area 117 in the sleeve 115, the end of the sleeve 115, and passages 129 in the valve housing 112. As previously described, as the fluid pressure is exhausted from the cylinders 98, the springs 109 will rotate the cam members 78 and 79 in the opposite direction to increase the volume output of the pump. The volume output of the pump will be increased a sufficient amount to meet the demands of the fluid system at rated pressure provided the demands of the system do not exceed the full capacity of the pump.

As can be seen from the above, the discharge pressure of the pump is controlled by the force exerted on the valve member 116 by the spring 124. It is thus possible to vary the discharge pressure of the pump by varying the force provided by the spring 124, and this is accomplished by loosening the nut 127 and threading the end cap 126 into or out of the housing 112 to increase or decrease, respectively, the force provided by the spring.

I claim:

1. A fluid-translating device comprising: a member having a plurality of openings therein defining cylinders extending in a longitudinal direction; a plurality of pistons longitudinally reciprocally mounted in said cylinders; a thrust cam having an arcuate cam surface mounted in driving relation to said pistons; said arcuate cam surface being a segment of a cylindrical surface formed about an axis angularly disposed with respect to the longitudinal dimension of said member; and means effecting relative rotation between said member and said thrust cam.

2. The device as set forth in claim 1 wherein each of said pistons includes a slipper universally pivotally mounted on a projecting end thereof, each of said slippers having first and second oppositely facing arcuate bearing surfaces, with each bearing surface being a segment of a cylindrical surface formed about an axis angularly disposed with respect to the longitudinal dimensions of said member, said first bearing surface being complementary to and in sliding engagement with said cam surface, and wherein said device further includes a return cam having an arcuate cam surface which is a segment of a cylindrical surface formed about an axis angularly disposed with respect to to the longitudinal dimension of said member, the arcuate cam surface of the return cam being complementary to and in sliding engagement with said second bearing surface. I

3. The device as set forth in claim 2 wherein said arcuate cam surfaces on said thrust and return cams and said first and second bearing surfaces on each of said slippers are formed about a common axis.

4. The device as set forth in claim 3 wherein said arcuate cam surface on said thrust cam and said second arcuate bearing surface are concave and said arcuate cam surface on said return cam and said first arcuate bearing surface are convex.

5. The device as set forth in claim 1 wherein said arcuate cam surface is a segment of a cylindrical surface formed about an axis which intersects and is perpendicular to the longitudinal axis of said member.

6. The device as set forth in claim 2 wherein the arcuate cam surface is concave and the cylinders converge away from the thrust cam to thereby minimize the maximum angle between the pistons and the cam surface.

7. A fluid-translating device comprising: a member having a plurality of openings therein defining cylinders extending in a longitudinal direction; a plurality of pistons reciprocally mounted in the cylinders and each having an end projecting from its respective cylinders; a plurality of slippers movably mounted on the projecting ends of the pistons and each having a bearing surface; a thrust cam having a cam surface mounted in driving engagement with the bearing surfaces of the slippers; the cam surface and bearing surfaces being complementary arcuate surfaces, one being concave and the other convex, and each being a segment of a cylindrical surface formed about an axis angularly disposed with respect to the longitudinal dimension of said member; and means effecting relative rotation between the member and the thrust cam.

8. The device as set forth in claim 7 wherein each of said slippers has a second bearing surface which faces away from said cam surface, and said device further includes a return having a cam surface in sliding engagement with said second bearing surface on each of said slippers.

9. The device as set forth in claim 7 wherein said arcuate cam surface is concave and the arcuate bearing surface on each of said slippers is convex and has a radius of curvature equal to the radius of curvature of the arcuate cam surface.

10. The device as set forth in claim 9 wherein said cylinders converge away from said thrust cam whereby the maximum angle between each piston and its associated slipper is minimized.

11. The device set forth in claim 9 wherein said means effecting relative rotation rotates said member about its longitudinal axis.

12. A fluid-translating device comprising: a pair of cylinder barrels mounted in end-to-end relation; each of the cylinder barrels having a plurality of openings therein defining cylinders extending in a longitudinal direction; valve means associated with the cylinder barrels; a plurality of pistons reciprocally mounted in the cylinders and having projecting ends; a plurality of slippers movably mounted on the projecting ends of the pistons and each having a first bearing surface; thrust 'cam means having cam surfaces mounted in driving engagement with the bearing surfaces of the slippers; the cam surfaces and bearing surfaces being complementary arcuate surfaces, one being concave and the other convex, and each being a segment of a cylindrical surface formed about an axis angularly disposed with respect to the longitudinal dimension of the cylinder barrels; and means effecting relative rotation between the pair of cylinder barrels and the thrust cam means.

13. The device as set forth in claim 12 wherein said thrust cam means includes first and second thrust cams mounted at the outer ends of said cylinder barrels, and each of said thrust cams has an arcuate cam surface.

14 The device as set forth in claim 13 wherein the arcuate cam surface on each thrust cam is concave and the arcuate bearing surface on each slipper is convex.

15. The device as set forth in claim 13 wherein the means effecting relative rotation rotates said cylinder barrels about their longitudinal axis, said valve means includes avalve plate positioned between the inner ends of said cylinder barrels and having supply and exhaust ports, the supply and exhaust ports extend through and are spaced about said valve plate, said thrust cams are mounted for limited oscillatory rotation about the axis of rotation of the cylinder barrels, said valve plate further includes a plurality of bores extending axially therethrough between the supply and exhaust ports, and said device further includes means linking said first and second thrust cams together to cause said thrust cams to rotate an equal amount and in opposite directions, whereby the displacement of said device can be varied by rotating said thrust earns.

16. The device as set forth in claim 15 further including means responsive to the fluid pressure in said exhaust ports controlling the oscillatory rotation of said thrust cams in accordance with variations in the fluid pressure in said exhaust ports.

17. The device as set forth in claim 13 wherein each of said slippers has a second bearing surface which faces away from the arcuate cam surface on the associated thrust cam, and said arcuate bearing surface of each slipper is maintained in sliding engagement with the arcuate cam surface on the associated thrust cam by return cams mounted at the outer ends of said cylinder barrels and engaging the second bearing surface on each of said slippers.

18. The device as set forth in claim 13 wherein a pair of return cams having arcuate cam surfaces are mounted at the outer ends of said cylinder barrels between said cylinder barrels and said thrust cams; each of the arcuate cam surfaces of the return carns being a segment of a cylindrical surface formed about the same axis about which the arcuate cam surface on the corresponding thrust cam is formed; the cam surfaces on said return cams facing the cam surfaces on said thrust cams; each of said slippers having a second arcuate bearing surface; the second arcuate bearing surfaces on each of said slippers being a segment of a cylindrical surface formed about the same axis about which the arcuate cam surface on the corresponding thrust cam is formed; and the second arcuate bearing surface of each slipper being in sliding engagement with the arcuate cam surface on the associated return cam.

19 The device as set forth in claim 18 wherein the arcuate cam surfaces on said thrust cams and the second arcuate hearing surfaces on said slippers are concave; and the arcuate bearing surfaces on said return cams and the first arcuate bearing surfaces on said slippers are convex.

20. The device as set forth in claim 19 wherein the means effecting relative rotation rotates said cylinder barrels about their longitudinal axis; said valve means includes a valve plate positioned between said cylinder barrels and provided with supply and exhaust ports extending axially therethrough and a plurality of bores extending therethrough between said supply and exhaust ports; each of said return cams is mounted in a fixed position with respect to the corresponding thrust cams; said thrust cams are mounted for limited oscillatory rotation about the axis of rotation of said cylinder barrels; and said device further includes means linking said thrust earns together for equal simultaneous oscillatory rotation in opposite directions, whereby the displacement of said device may be varied by rotating said thrust cams.

21 The device as set forth in claim 20, and further including means responsive to the fluid pressure in said exhaust ports controlling the oscillatory rotation of said thrust cams in accordance with variations of the fluid pressure in said exhaust ports.

22. The device as set forth in claim 21 wherein said means controlling the oscillatory rotation of said thrust cams includes means resiliently urging said thrust cams in one direction, a hydraulic cylinder means responsive to fluid pressure supplied thereto to force said thrust cams in the other direction, fluidconducting means between said hydraulic cylinder means and said exhaust ports, valve means interposed in said fluid conducting means and operative to prevent the flow of fluid pressure from said exhaust ports to said hydraulic cylinder when the fluid pressure in said exhaust ports is below a predetermined value and to allow the flow of fluid to said hydraulic cylinder from said exhaust ports when the fluid pressure in said exhaust ports is above said predetermined value.

23. A fluid-translating apparatus comprising: a hollow housing; a shaft joumaled for rotation within said housing and extending from one end thereof; a valve plate having supply and exhaust ports mounted within said housing, freely encircling said shaft, and positioned substantially perpendicular to said shaft: a cylinder barrel mounted on said shaft for rotation therewith and having an end face in engagement with said valve plate; a thrust cam having an arcuate cam surface mounted along the axis of rotation of said shaft at the end of said cylinder barrel; said arcuate cam surface being a segment of a cylindrical surface formed about an axis which intersects the axis of rotation of said shaft; a plurality of generally axially extending cylinders providedwithin said cylinder barrel; and a plurality of pistons reciprocally mounted within said cylinders and having projecting ends in driving relation with said cam surface.

24. The fluid-translating apparatus as set forth in claim 23 wherein said cylinder barrel is provided with a bore along the axis of rotation thereof, said bore having a diameter greater than the diameter of said shaft; a drive flange is keyed to said shaft for rotation therewith; and said drive flange includes means engageable with means on the outer periphery of said cylinder barrel, whereby said shaft and cylinder barrel will rotate together.

25. The fluid-translating apparatus as set forth in claim 24 further including: a pressure chamber between said cylinder barrel and said drive flange and means establishing fluid communication between said pressure chamber and one of said supply and exhaust ports, whereby during operation, the fluid pressure in said one of said supply and exhaust ports will maintain the end face of said cylinder barrel in engagement with said valve plate.

26. A fluid-translating device comprising: a hollow housing; a drive shaft rotatably journaled within said housing and extending from one end thereof; a valve plate mounted in said housing transversely to said shaft and having a central bore through which said shaft extends; said valve plate having a plurality of supply and exhaust ports extending therethrough; first.

and second cylinder barrels mounted on said shaft adjacent the sides of said valve plate; each of said cylinder barrels having a plurality of substantially axially extending cylinders; first and second drive flanges mounted on said shaft adjacent the outer ends of said cylinder barrels; each of said flanges being fixed to said shaft for rotation therewith; each of said drive flanges having means loosely engaging the corresponding cylinder barrel in driving relation whereby said shaft, cylinder barrels and drive flanges will rotate as a single unit; a plurality of axial clearance openings provided in each of said drive flanges; said clearance openings corresponding in number and location to said cylinders; a plurality of pistons reciprocally mounted in said cylinders and having one end projecting therefrom and through said clearance openings; and first and ,4 second thrust cams positioned adjacent said drive flange and having arcuate cam surfaces engaging the projecting ends of the corresponding pistons; each of said cam surfaces being a segment of a cylindrical surface formed about an axis which intersects the axis of rotation of said shaft.

27. The fluid-translating apparatus as set forth in claim 26 further including: a pressure chamber between each of said cylinder barrels and the corresponding drive flange; and means establishing fluid communication between said pressure chambers and one of said supply and exhaust ports; whereby the fluid pressure in said one of said supply and exhaust ports will hold said cylinder barrels in engagement with said valve plate.

28. The fluid-translating apparatus as set forth in claim 26 wherein each of said thrust cams is mounted for oscillatory rotation through 45"; said valve plate is provided with a plurality of through bores between the supply and exhaust ports, and said device further includes means linking said thrust cams together for equal simultaneous rotation in opposite directions whereby the displacement of said apparatus may be varied by rotating said cams.

29. The fluid-translating apparatus as set forth in claim 28 further including means responsive to the fluid pressure in one of said supply and exhaust ports controlling the oscillatory rotation of saidthrust cams whereby the displacement of said apparatus will be varied in response to pressure variations in said one of said supply and exhaust ports.

30. The fluid-translating apparatus as set forth in claim 29 wherein said means controlling the rotation of said thrust cams includes: an extensible and retractable hydraulic actuator having a cylinder end and a rod end, said cylinder end being mounted on said housing and said rod end being secured to one of said thrust cams; means establishing fluid communication between one of said supply and exhaust ports and the cylinder end of said actuator; and valve means interposed in said last-mentioned means, said valve means being responsive to the fluid pressure in said one of said supply and exhaust ports to allow the flow of fluid from said one of said supply and exhaust ports to said cylinder end of said actuator when the fluid pressure within said one of said supply and exhaust ports exceeds a predetermined value and to exhaust the fluid pressure from said cylinder end of said actuator when the fluid pressure in said one of said supply and exhaust ports is less than said predetermined value.

31. A fluid-translating device comprising: a pair of cylinder barrels mounted in end-to-end relation; each of said cylinder barrels having a plurality of openings therein defining cylinders extending in a longitudinal direction; valve means associated with said cylinder barrels; a plurality of pistons reciprocally mounted in said cylinders; thrust cam means having arcuate cam surfaces mounted in driving relation to said pistons; each of said arcuate cam surfaces being a segment of a cylindrical surface formed about an axis which is angularly disposed with respect to the longitudinal dimension of said cylinder barrels; and means effecting relative rotation between said pair of cylinder barrels and said thrust cam means.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,596,558 Dated 3 August 1971 Inventor) Richard Arthur Wittren It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

line 15, cancel "to" (2nd occ.); line 31,

Column 8, change "2" to 5 line 39, change "cylinders" to cylinder line 52, before "having" insert cam Signed and sealed this 18th day of April 1972.

(SEAL) A t test:

EDWARD M.FLETCHI1;R,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 'ORM PO-1050 [10-59) & u s GOVERNMENT PRINTING orncr 1909 0-395-334 

1. A fluid-translating device comprising: a member having a plurality of openings therein defining cylinders extending in a longitudinal direction; a plurality of pistons longitudinally reciprocally mounted in said cylinders; a thrust cam having an arcuate cam surface mounted in driving relation to said pistons; said arcuate cam surface being a segment of a cylindrical surface formed about an axis angularly disposed with respect to the longitudinal dimension of said member; and means effecting relative rotation between said member and said thrust cam.
 2. The device as set forth in claim 1 wherein each of said pistons includes a slipper universally pivotally mounted on a projecting end thereof, each of said slippers having first and second oppositely facing arcuate bearing surfaces, with each bearing surface being a segment of a cylindrical surface formed about an axis angularly disposed with respect to the longitudinal dimensions of said member, said first bearing surface being complementary to and in sliding engagement with said cam surface, and wherein said device further includes a return cam having an arcuate cam surface which is a segment of a cylindrical surface formed about an axis angularly disposed with respect to to the longitudinal dimension of said member, the arcuate cam surface of the return cam being complementary to and In sliding engagement with said second bearing surface.
 3. The device as set forth in claim 2 wherein said arcuate cam surfaces on said thrust and return cams and said first and second bearing surfaces on each of said slippers are formed about a common axis.
 4. The device as set forth in claim 3 wherein said arcuate cam surface on said thrust cam and said second arcuate bearing surface are concave and said arcuate cam surface on said return cam and said first arcuate bearing surface are convex.
 5. The device as set forth in claim 1 wherein said arcuate cam surface is a segment of a cylindrical surface formed about an axis which intersects and is perpendicular to the longitudinal axis of said member.
 6. The device as set forth in claim 2 wherein the arcuate cam surface is concave and the cylinders converge away from the thrust cam to thereby minimize the maximum angle between the pistons and the cam surface.
 7. A fluid-translating device comprising: a member having a plurality of openings therein defining cylinders extending in a longitudinal direction; a plurality of pistons reciprocally mounted in the cylinders and each having an end projecting from its respective cylinders; a plurality of slippers movably mounted on the projecting ends of the pistons and each having a bearing surface; a thrust cam having a cam surface mounted in driving engagement with the bearing surfaces of the slippers; the cam surface and bearing surfaces being complementary arcuate surfaces, one being concave and the other convex, and each being a segment of a cylindrical surface formed about an axis angularly disposed with respect to the longitudinal dimension of said member; and means effecting relative rotation between the member and the thrust cam.
 8. The device as set forth in claim 7 wherein each of said slippers has a second bearing surface which faces away from said cam surface, and said device further includes a return having a cam surface in sliding engagement with said second bearing surface on each of said slippers.
 9. The device as set forth in claim 7 wherein said arcuate cam surface is concave and the arcuate bearing surface on each of said slippers is convex and has a radius of curvature equal to the radius of curvature of the arcuate cam surface.
 10. The device as set forth in claim 9 wherein said cylinders converge away from said thrust cam whereby the maximum angle between each piston and its associated slipper is minimized.
 11. The device set forth in claim 9 wherein said means effecting relative rotation rotates said member about its longitudinal axis.
 12. A fluid-translating device comprising: a pair of cylinder barrels mounted in end-to-end relation; each of the cylinder barrels having a plurality of openings therein defining cylinders extending in a longitudinal direction; valve means associated with the cylinder barrels; a plurality of pistons reciprocally mounted in the cylinders and having projecting ends; a plurality of slippers movably mounted on the projecting ends of the pistons and each having a first bearing surface; thrust cam means having cam surfaces mounted in driving engagement with the bearing surfaces of the slippers; the cam surfaces and bearing surfaces being complementary arcuate surfaces, one being concave and the other convex, and each being a segment of a cylindrical surface formed about an axis angularly disposed with respect to the longitudinal dimension of the cylinder barrels; and means effecting relative rotation between the pair of cylinder barrels and the thrust cam means.
 13. The device as set forth in claim 12 wherein said thrust cam means includes first and second thrust cams mounted at the outer ends of said cylinder barrels, and each of said thrust cams has an arcuate cam surface. 14 The device as set forth in claim 13 wherein the arcuate cam surface on each thrust cam is concave and the arcuate bearing surface on each slipper is convex.
 15. The device as set forth in claim 13 wHerein the means effecting relative rotation rotates said cylinder barrels about their longitudinal axis, said valve means includes a valve plate positioned between the inner ends of said cylinder barrels and having supply and exhaust ports, the supply and exhaust ports extend through and are spaced about said valve plate, said thrust cams are mounted for limited oscillatory rotation about the axis of rotation of the cylinder barrels, said valve plate further includes a plurality of bores extending axially therethrough between the supply and exhaust ports, and said device further includes means linking said first and second thrust cams together to cause said thrust cams to rotate an equal amount and in opposite directions, whereby the displacement of said device can be varied by rotating said thrust cams.
 16. The device as set forth in claim 15 further including means responsive to the fluid pressure in said exhaust ports controlling the oscillatory rotation of said thrust cams in accordance with variations in the fluid pressure in said exhaust ports.
 17. The device as set forth in claim 13 wherein each of said slippers has a second bearing surface which faces away from the arcuate cam surface on the associated thrust cam, and said arcuate bearing surface of each slipper is maintained in sliding engagement with the arcuate cam surface on the associated thrust cam by return cams mounted at the outer ends of said cylinder barrels and engaging the second bearing surface on each of said slippers.
 18. The device as set forth in claim 13 wherein a pair of return cams having arcuate cam surfaces are mounted at the outer ends of said cylinder barrels between said cylinder barrels and said thrust cams; each of the arcuate cam surfaces of the return cams being a segment of a cylindrical surface formed about the same axis about which the arcuate cam surface on the corresponding thrust cam is formed; the cam surfaces on said return cams facing the cam surfaces on said thrust cams; each of said slippers having a second arcuate bearing surface; the second arcuate bearing surfaces on each of said slippers being a segment of a cylindrical surface formed about the same axis about which the arcuate cam surface on the corresponding thrust cam is formed; and the second arcuate bearing surface of each slipper being in sliding engagement with the arcuate cam surface on the associated return cam. 19 The device as set forth in claim 18 wherein the arcuate cam surfaces on said thrust cams and the second arcuate bearing surfaces on said slippers are concave; and the arcuate bearing surfaces on said return cams and the first arcuate bearing surfaces on said slippers are convex.
 20. The device as set forth in claim 19 wherein the means effecting relative rotation rotates said cylinder barrels about their longitudinal axis; said valve means includes a valve plate positioned between said cylinder barrels and provided with supply and exhaust ports extending axially therethrough and a plurality of bores extending therethrough between said supply and exhaust ports; each of said return cams is mounted in a fixed position with respect to the corresponding thrust cams; said thrust cams are mounted for limited oscillatory rotation about the axis of rotation of said cylinder barrels; and said device further includes means linking said thrust cams together for equal simultaneous oscillatory rotation in opposite directions, whereby the displacement of said device may be varied by rotating said thrust cams. 21 The device as set forth in claim 20, and further including means responsive to the fluid pressure in said exhaust ports controlling the oscillatory rotation of said thrust cams in accordance with variations of the fluid pressure in said exhaust ports.
 22. The device as set forth in claim 21 wherein said means controlling the oscillatory rotation of said thrust cams includes means resiliently urging said thrust cams in one direction, a hydraulic cylinder means responsive to Fluid pressure supplied thereto to force said thrust cams in the other direction, fluid-conducting means between said hydraulic cylinder means and said exhaust ports, valve means interposed in said fluid conducting means and operative to prevent the flow of fluid pressure from said exhaust ports to said hydraulic cylinder when the fluid pressure in said exhaust ports is below a predetermined value and to allow the flow of fluid to said hydraulic cylinder from said exhaust ports when the fluid pressure in said exhaust ports is above said predetermined value.
 23. A fluid-translating apparatus comprising: a hollow housing; a shaft journaled for rotation within said housing and extending from one end thereof; a valve plate having supply and exhaust ports mounted within said housing, freely encircling said shaft, and positioned substantially perpendicular to said shaft: a cylinder barrel mounted on said shaft for rotation therewith and having an end face in engagement with said valve plate; a thrust cam having an arcuate cam surface mounted along the axis of rotation of said shaft at the end of said cylinder barrel; said arcuate cam surface being a segment of a cylindrical surface formed about an axis which intersects the axis of rotation of said shaft; a plurality of generally axially extending cylinders provided within said cylinder barrel; and a plurality of pistons reciprocally mounted within said cylinders and having projecting ends in driving relation with said cam surface.
 24. The fluid-translating apparatus as set forth in claim 23 wherein said cylinder barrel is provided with a bore along the axis of rotation thereof, said bore having a diameter greater than the diameter of said shaft; a drive flange is keyed to said shaft for rotation therewith; and said drive flange includes means engageable with means on the outer periphery of said cylinder barrel, whereby said shaft and cylinder barrel will rotate together.
 25. The fluid-translating apparatus as set forth in claim 24 further including: a pressure chamber between said cylinder barrel and said drive flange and means establishing fluid communication between said pressure chamber and one of said supply and exhaust ports, whereby during operation, the fluid pressure in said one of said supply and exhaust ports will maintain the end face of said cylinder barrel in engagement with said valve plate.
 26. A fluid-translating device comprising: a hollow housing; a drive shaft rotatably journaled within said housing and extending from one end thereof; a valve plate mounted in said housing transversely to said shaft and having a central bore through which said shaft extends; said valve plate having a plurality of supply and exhaust ports extending therethrough; first and second cylinder barrels mounted on said shaft adjacent the sides of said valve plate; each of said cylinder barrels having a plurality of substantially axially extending cylinders; first and second drive flanges mounted on said shaft adjacent the outer ends of said cylinder barrels; each of said flanges being fixed to said shaft for rotation therewith; each of said drive flanges having means loosely engaging the corresponding cylinder barrel in driving relation whereby said shaft, cylinder barrels and drive flanges will rotate as a single unit; a plurality of axial clearance openings provided in each of said drive flanges; said clearance openings corresponding in number and location to said cylinders; a plurality of pistons reciprocally mounted in said cylinders and having one end projecting therefrom and through said clearance openings; and first and second thrust cams positioned adjacent said drive flange and having arcuate cam surfaces engaging the projecting ends of the corresponding pistons; each of said cam surfaces being a segment of a cylindrical surface formed about an axis which intersects the axis of rotation of said shaft.
 27. The fluid-translating apparatus as set forth in claim 26 further including: a pressure chamber between eAch of said cylinder barrels and the corresponding drive flange; and means establishing fluid communication between said pressure chambers and one of said supply and exhaust ports; whereby the fluid pressure in said one of said supply and exhaust ports will hold said cylinder barrels in engagement with said valve plate.
 28. The fluid-translating apparatus as set forth in claim 26 wherein each of said thrust cams is mounted for oscillatory rotation through 45*; said valve plate is provided with a plurality of through bores between the supply and exhaust ports, and said device further includes means linking said thrust cams together for equal simultaneous rotation in opposite directions whereby the displacement of said apparatus may be varied by rotating said cams.
 29. The fluid-translating apparatus as set forth in claim 28 further including means responsive to the fluid pressure in one of said supply and exhaust ports controlling the oscillatory rotation of said thrust cams whereby the displacement of said apparatus will be varied in response to pressure variations in said one of said supply and exhaust ports.
 30. The fluid-translating apparatus as set forth in claim 29 wherein said means controlling the rotation of said thrust cams includes: an extensible and retractable hydraulic actuator having a cylinder end and a rod end, said cylinder end being mounted on said housing and said rod end being secured to one of said thrust cams; means establishing fluid communication between one of said supply and exhaust ports and the cylinder end of said actuator; and valve means interposed in said last-mentioned means, said valve means being responsive to the fluid pressure in said one of said supply and exhaust ports to allow the flow of fluid from said one of said supply and exhaust ports to said cylinder end of said actuator when the fluid pressure within said one of said supply and exhaust ports exceeds a predetermined value and to exhaust the fluid pressure from said cylinder end of said actuator when the fluid pressure in said one of said supply and exhaust ports is less than said predetermined value.
 31. A fluid-translating device comprising: a pair of cylinder barrels mounted in end-to-end relation; each of said cylinder barrels having a plurality of openings therein defining cylinders extending in a longitudinal direction; valve means associated with said cylinder barrels; a plurality of pistons reciprocally mounted in said cylinders; thrust cam means having arcuate cam surfaces mounted in driving relation to said pistons; each of said arcuate cam surfaces being a segment of a cylindrical surface formed about an axis which is angularly disposed with respect to the longitudinal dimension of said cylinder barrels; and means effecting relative rotation between said pair of cylinder barrels and said thrust cam means. 